1. Field of the Invention
This invention relates to electronic watt-hour meters in which electronic circuits multiply the current by the voltage and integrate the product to determine the electrical energy. More particularly, the invention relates to electronic circuits which combine the multiplication and integration functions in an electronic watt-hour meter.
2. Background of the Invention
Electronic circuits are known for measuring electrical energy. Typically, the current is multiplied by the voltage in a multiplication circuit. The product of this multiplication, which is the power, is then integrated by an integrating circuit to generate a quantity proportional to electrical energy. When the output of the integrator reaches a preset amount, a pulse is generated and the integrator is reset. Each output pulse therefor represents a predetermined unit of electrical energy. The output pulses are accumulated to record total energy consumption. It is common to have the integrator alternate the direction of integration on successive output pulses to reduce the effects of offset in the operational amplifier typically used in the integrator.
It is common to electronically perform the multiplication function by generating a pulse width modulated signal modulated by either the load current or voltage. This pulse width modulated signal, having a duty cycle proportional to the magnitude of the modulating parameter, is used to gate a voltage signal proportional to the other load parameter to the integrator. Positive values of this voltage signal are gated to the integrator when the pulse width modulated signal has one polarity, and negative values of the voltage signal are gated to the integrator for the other polarity of the pulse width modulated signal. The difference in the charge supplied to the integrator for the two polarities of the pulse width modulated signal represents the energy.
U.S. Pat. No. 4,596,951 discloses such an electronic watt-hour meter. In this patent, a mutual inductance current transducer is used to provide a signal proportional to di/dt, the time derivative of the metered circuit current. This di/dt signal is summed with a square wave clock signal and applied to an integrator. The output of the integrator is applied to a comparator which generates the pulse width modulated signal having a duty cycle proportional to the level of the current. The two signals of opposite polarity proportional to the voltage are generated in the secondary winding of a potential transformer. This arrangement requires that the potential transformer be exactly balanced, otherwise an ac ripple is present in the output. It also requires two matched resistors for converting the voltage signals to currents. Again, imbalance in these resistors will cause errors in the output.
U.S. Pat. No. 4,682,102 discloses a solid state watt-hour meter with switched capacitor integration. This meter has separate multiplication and integration circuits, however, the input resistor to the integrator is replaced by switched capacitors. The purpose of using switch capacitor integration is to eliminate the need for resistors and capacitors of high accuracy in the integrator so that the entire watt-hour meter may be implemented on an integrated circuit chip. It is recognized that the accuracy in the switched capacitor integration is related to the ratio of the capacitances in the integrator and not their absolute value, and to the magnitude of a reference voltage and the accuracy of the clock signal.
There remains a need, however, for an electronic watt-hour meter which does not require opposite polarity voltage sources and does not require matched components.
It is another object of the invention to provide an electronic watt-hour meter with reduced circuitry.
It is yet another object of the invention to provide an electronic watt-hour meter in which the multiplication and integration are carried out by the same components.
It is an overall object of the invention to provide an electronic watt-hour meter in accordance with the above objects, which has high reliability and is economical to produce.
It is another object of the invention to provide such as electronic watt-hour meter which may be implemented on integrated circuit chips.